A fuel cell has an electrolyte and a pair of electrodes separated by the electrolyte. In inside of a fuel cell, a fuel like hydrogen is supplied to one of the two electrodes and an oxidant like oxygen is supplied to the other electrode, suggesting that chemical energy including oxidation of a fuel is converted into electric energy. Hydrogen ion (that is, proton) can pass through the electrolyte but reactant gases (that is, hydrogen and oxygen) cannot. In general, laminated fuel cell contains numbers of fuel cells and each of the fuel cell has one electrolyte and a pair of electrodes separated by the electrolyte.
Fuel cells are classified into solid oxide electrolyte fuel cell, molten carbonate fuel cell, phosphoric acid fuel cell and polymer electrolyte membrane fuel cell, according to the electrolyte included in them. Polymer electrolyte membrane fuel cell contains polymer electrolyte membrane as its electrolyte, and thus, has high energy density and efficiency as well as other advantages such as fast starting and stop owing to its low operating temperature.
In order to be used for a fuel cell, an electrolyte has to have high proton conductivity, low methanol permeability and stability on operating temperature. In the case of Nafion polymer electrolyte membrane, provided by Du Pont, USA, it cannot be commonly used because of high unit price and low thermal stability, although it has excellent mechanical and chemical stability and high proton conductivity. Thus, studies have been vigorously undertaking to overcome the above barriers to produce a polymer electrolyte membrane.
As of today, the studies have been directed to develop a hydrocarbon polymer or a fluoride/hydrocarbon mixed polymer electrolyte membrane, rather than a fluoride polymer electrolyte membrane (Li, et al., Chemistry of materials, ACS, 2003, 15, 4896-4915).
One of the most representative hydrocarbon polymer electrolyte membranes was produced by doping phosphoric acid (H3PO4) into an alkali polymer polybenzimidazole, which has very low fuel permeability, moderate electrolyte price, excellent mechanical properties, oxidation resistance and thermal stability (Wainwright, et al., Journal of the Electrochemical Society, ECS, 1995, 142, L121; Li, et al., Solid State Ionics, Elservier, 2004, 168, 177-185; Asensio, et al., Journal of Polymer Science, 2002, 40, 3703-3710; Asensio, et al., Journal of the Electrochemical Society, ECS, 2004, 151, A304-A310; Kim, et al., Macromolecular Rapid Communications, Wiley, 2004, 25, 894-897).
Nevertheless, the polybenzimidazole polymer electrolyte membrane prepared by doping phosphoric acid has inappropriate proton conductivity, not sufficient enough to replace Nafion.